Altered DNA methylation patterns, characterized by genome-wide hypomethylation in combination with CpG island hypermethylation, are one of the hallmarks of human tumor cells. Aberrant promoter /hypemethylation leads to silencing of tumor suppressor genes, while aberrant hypomethylation of repetitive sequences leads to genomic instability. The mechanisms that regulate de novo methylation and the de novo DNA methyltransferases DNMT3 A and DNMT3B remain almost totally unknown. Unlike DNMT3 A, DNMT3B is frequently over-expressed in tumor cells and paradoxically this can lead to both hypermethylation and hypomethylation events depending on the particular splice variant expressed. DNMT3B is also mutated in the rare disorder known as ICF syndrome, the only known genetic disease due to mutation of a DNA methyltransferase. The central hypothesis to be tested in this application is that DNMT3B is targeted to specific regions of the genome via interactions with chromatin-associated proteins and perturbing these interactions leads to abnormal methylation patterns and tumorigenesis. The three aims of this proposal are designed to test this hypothesis. In aim 1, the enzymatic and DNA binding properties of DNMT3B will be analyzed in vitro on naked DNA and chromatin substrates in the presence and absence of known DNMT3B interacting partners. Our second aim entails functionally characterizing a novel DNMT3B complex that we have identified which contains chromatin remodeling enzymes and condensin proteins. Lastly, we aim to identify and characterize DNMT3B target sequences using two powerful and complementary whole-genome-based microarray techniques. Addressing these questions will contribute to our fundamental understanding of how cellular DNA methylation patterns, particularly those mediated by DNMT3B, are established and maintained in cells. This is expected to positively affect human health by allowing for the development of novel therapies that will target aberrant epigenetic changes and provide additional new targets for pharmacologic intervention.